RFR: 8371914: PNG defines in CFLAGS can cause compilation errors with external libpng

[Erik Joelsson]

On Fri, 14 Nov 2025 14:48:14 GMT, Kurt Miller <kurt at openjdk.org> wrote:

> When building with external libpng the PNG defines added to CFLAGS may conflict with the installed libpng header pnglibconf.h resulting in compilation errors. For example on OpenBSD/aarch64:
> 
> `/usr/local/include/pnglibconf.h:207:9: error: 'PNG_ARM_NEON_OPT' macro redefined [-Werror,-Wmacro-redefined]`
> 
> This moves all `-DPNG_*` into the case where internal libpng is used. This has only been tested with the tier1 github actions so needs to be checked on Linux/ppc and AIX.

I think this looks ok, but someone from client should weigh in too.

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Marked as reviewed by erikj (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28324#pullrequestreview-3474397386